Many medical intravascular devices are used either temporarily or permanently inside the human body. An example of such an intravascular device includes a stent for use in, for instance, coronary angioplasty. Stents are small metal scaffolds used to mechanically hold open and support constricted coronary arteries. For proper positioning, stents may need to be visualized during and after deployment using imaging techniques such as x-ray radiography and x-ray fluoroscopy. However, due to the nature of the materials used to construct these intravascular devices and their small size, visualization of these devices can often be poor or non-existent.
Certain “radiopaque” materials are known to be more effective in stopping energetic x-ray photons, and as a result, are more readily visualized during, for instance, x-ray imaging. However, incorporation of these radiopaque materials, including ones that are biocompatible, into the device substrate material can have an undesirable effect on other device characteristics, such as mechanical performance.
Traditional methods for adding opacity to a device include the use of metal bands, electrochemical deposition (i.e., electroplating), or coatings. In the case of metal bands or disks having radiopaque material, the bands or disks may be crimped, swaged, pressed or glued on to the device at selected points. However, bands have the potential for becoming loose, shifting, or even falling off. Moreover, bands may also cause abrasion to the intima (i.e., the lining of a vessel wall) during insertion of the device, especially if the bands have sharp edges or outward projections. The physiological response can often be a reclosure of the lumen, thereby negating the effect of the device. Additionally, cellular debris can be trapped between the intravascular device and the band, and the edges of the band can serve as a site for thrombus formation.
Alternatively, a metal coating can be used as a marker and can be applied on to an intravasuclar device using chemical vapor deposition (CVD), physical vapor deposition (PVD), or electroplating. However, for the range of thickness required to make the coating x-ray opaque, CVD and conventional PVD methods do not appear to provide a coating which can exhibit sufficient adhesion to the surface of the device, especially a stainless steel substrate surface, to be reliable in a medical device application.
On the other hand, electroless and/or electroplated coatings are often porous, and can present a biocompatibility problem, since the porous coating can act to entrap the plating chemicals. For devices constructed from, for instance, titanium alloys, embrittlement caused by the electroplating process can occur to significantly alter the mechanical properties and thus the function of the device.
Ion beam assisted deposition (IBAD) of radiopaque materials has been used and can improve the adhesion of coatings to the substrate surface. IBAD employs conventional PVD to create a vapor of atoms of, for instance, a noble metal that coats the surface of the substrate, while simultaneously bombarding the substrate surface with ions at energies, typically in the range of 0.8 to 1.5 keV, to impact and condense the metal atoms on the substrate surface. An independent ion source is used as the source of ions.
Coatings produced by traditional IBAD, however, are costly. When evaporating, atoms of expensive noble metal are emitted over a large solid angle compared to that subtended by the device or devices being coated, thus requiring a costly reclaiming process. Moreover, because an evaporator uses a molten metal, it must be located upright on the floor of the deposition chamber to avoid spilling, thereby restricting the size and configuration of the chamber and the devices being coated. Additionally, evaporators cannot deposit mixtures of alloys effectively because of the differences in the alloy components' evaporation rates. As such, the composition of the resulting coating constantly changes.
Furthermore, with IBAD, the flux (i.e. stream) of bombarding ions and evaporant (i.e., atoms of metal being deposited) approach the substrate from different directions. To this end, the energy from the bombarding ions transferred to the evaporant atoms varies depending on the extent to which the two streams overlap. As a result, the growth mechanism of the coating can be inconsistent, and uniform coating properties are difficult to achieve even over the same device.